The invention relates generally to microfabricated devices and more particularly to microfluidic devices for chemical and biological analysis, and chemical synthesis.
Microfluidic technology may be utilized to create systems that can perform chemical and biological analysis, and chemical synthesis on a much smaller scale than previous laboratory equipment and techniques. Microfluidic systems offer the advantages of requiring a smaller sample of analyte or reagent for analysis or synthesis, and dispensing a smaller amount of waste materials. Since the testing or combining is self-contained within the microfluidic system, analysis or synthesis can be performed in virtually any location inside or outside of the laboratory.
The microfluidic systems may be used for analytical and fine chemistry, biological sciences, clinical testing, combinatorial synthesis, environmental or forensic testing, and the like. Microfluidic systems for analysis, chemical and biological processing, and sample preparation may include some combination of the following elements: pre- and post-processing fluidic handling components, microfluidic-to-system interface components, electrical and electronic components, environmental control components, and data analysis components. A popular use of microfluidic systems is in the analysis of DNA molecules for testing infectious or genetic diseases or screening for genetic defects. Another popular use is in forensic sciences where immediate results of blood samples may be obtained.
In addition to the reduction of the microfluidic component down to the size of a xe2x80x9cchipxe2x80x9d (i.e., a semiconductor die), recent advances have allowed the simultaneous execution of multiple tasks on a single component. The capability of simultaneously performing multiple tasks has greatly enhanced the utility of microfluidic devices. Moreover, the time required to obtain the desired results is reduced.
The general principle behind a microfluidic device is that all the elements of the device are reduced to a microscopic scale. These elements may include fluid reservoirs, channels, testing regions, mixing chambers, etc. Each element is generally fabricated on the micron or submicron scale. For example, typical channels or regions have at least one cross-sectional dimension in the range of about 0.1 microns to about 500 microns.
FIG. 1 illustrates a conventional microfluidic system 10 fabricated on a substrate 12. The microfluidic substrate is made of a material such as polymer, glass, silicon, or ceramic. Polymers are the preferred substrate materials, with polyimide being the most preferred. Polymer materials that are particularly suitable include materials selected from the following classes: polyimide, PMMA, polycarbonate, polystyrene, polyester, polyamide, polyether, polyolefin, and mixtures of these materials.
The exemplary microfluidic system 10 is a planar device that includes an internal region 14 having input/output ports 16 and 18 and further includes an internal separation channel 20 having input/output ports 22 and 24. The internal region 14 and the separation channel 20 are shown as dashed lines, since they are formed within the substrate 12 of the microfluidic system 10. The dashed lines are interrupted at the intersection of the channel from region 14 with the channel from the separation channel 20, because the two channels intersect. Other configurations are possible and may have, for example, multiple internal regions, additional input and output ports, and a network of channels located within a substrate of a microfluidic system. The term xe2x80x9cinternal regionxe2x80x9d is used herein to describe a generally enclosed portion of the microfluidic system in which particular sample preparation processes are performed. Such processes include, but are not limited to, mixing, labeling, testing, filtering, extracting, precipitating, digesting, synthesizing and the like. Movement of the subject material within the device is generally facilitated by manipulation of an external force.
Performed within a typical microfluidic system are a number of tests, wherein the subject material can be processed in either a serial or parallel fashion depending on the individual test requirements. In the process of performing the tests, it is possible that the result from an earlier test will be used to determine which subsequent test will be performed on the subject material within the same microfluidic system. For example, if the result in Test Area #1 is positive, then the subject material will be directed to Test Area #2, where a subsequent analysis is performed based on the results of Test Area #1. Conversely, if the result of Test Area #1 is negative, then the subject material will be directed to Test Area #3. Accordingly, a method is needed for steering the subject material through a network of fluidic channels in response to the initial test result.
One known technique which attempts to steer the subject material to the appropriate testing region as a function of the prior test result involves having an external port at each decision point, so that an external force would be utilized to steer the subject material in one of two or more directions. However, in the case of a cascade of tests with even a small number of decisions, the number of required external ports is large. Large numbers of external fluidic ports are troublesome, as each fluidic port needs to be routed to the decision point independently of other ports.
Another known technique for routing the subject material involves using valves that extend and retract through the microfluidic device. Unfortunately, this technique requires moving mechanical parts that are often susceptible to failure.
Consequently, what is needed is a microfluidic system and a method for steering subject material to its appropriate testing region without the use of external ports or moving parts.
The present invention is a microfluidic system for directing an analyte, reagent, or similar subject material to a next region of interest, which may be a testing region, detecting region, controlling region, reaction region or the like. The microfluidic system is fabricated on and within a substrate comprising a network of channels and gas generators that are strategically located along the network of channels. As the gas generators are activated, the gas molecules contained within the gas generators expand and push the subject material along selected channels of the networks of channels. By strategically activating a particular gas generator, the subject material can be steered along a desired channel to its appropriate location of interest.
The microfluidic system may be formed using integrated circuit fabrication techniques, such as photolithographic processes, wet or dry chemical etching, or laser ablation. Alternatively, traditional machining techniques may be used. The microfluidic system may also be fabricated by indirect means such as injection molding, hot embossing, casting, or other processes that utilize a mold or patterned tool to form the features of the system. The microfluidic substrate is made of a material such as polymer, glass, silicon, metal, or ceramic. A polymer such as polyimide or polymethylmethacrylate (PMMA) is preferred.
While the microfluidic system is described as including a substrate, this is not critical to the invention. Rather, the microfluidic system can be fabricated on or within a body, housing and supporting structure, and the like, without diverting from the scope of the invention.
In the preferred embodiment, the gas generator that is utilized to direct the gas for manipulating the subject material in the channels includes microscopic resistors that are electrically activated. As current passes through a resistor, electrical energy is transferred into thermal energy. Each resistor is adjacent to a gas-forming chamber. Since the chamber is at a lower temperature than the resistors, there is a transfer of heat from the resistors to the chamber. There is a relatively large class of compounds that will decompose from one of a liquid state or a solid state to a gaseous state when heat is applied. For example, sodium azide (NaN3) will decompose upon application of heat to generate pure nitrogen (N2) gas. Similarly, most alkali bicarbonates (e.g., sodium bicarbonate) will generate carbon dioxide gas (CO2) upon thermal decomposition. Any one of these compounds can be used in the gas-forming chambers. However, identifying these compounds is not intended to limit the scope of the invention. Instead, the identifications are intended to serve as examples of commonly used compounds for chemical reactions to generate gaseous products.
Upon thermal decomposition of the gas-generating compounds, the pressure within a particular gas generator increases as a result of the volume expansion of the gas molecules. Since the volumes involved in the microfluidic system are small, the amount of gas required to steer the subject material along a selected channel is correspondingly small. Accordingly, only a minuscule amount of gas-generating compound is required to be decomposed to generate sufficient gas (measured in nanoliters) to steer the subject material to the next testing region. The typical amount of gas-generating compound is generally in the order of picomoles. While the actual amount of gas-generating compounds necessary to generate one nanoliter is a function of the actual reaction, in the case of sodium bicarbonate, approximately five nanograms of material will generate one nanoliter of gas.
The microfluidic system includes the substrate with internal features that include the gas-forming chambers, microfluidic channels, microfluidic compartments, and microfluidic flow control elements other than the gas chambers. Therefore, the microfluidic system may include known features such as capillary channels, separation channels, and detection channels. The microfluidic system is designed such that the subject material can be processed in either a serial or parallel fashion, depending on the desired testing. In the preferred embodiment, the result of an initial test (e.g., Test #1) determines which of the alternative subsequent tests (e.g., Test #2 or Test #3) is to be performed. Accordingly, the appropriate generator that is strategically located along the network of channels is activated to direct the subject material to its desired location in response to the result of the initial test. In an alternative embodiment, activation of a gas generator is made in response to an external control, such as a computer program or a human operator.
As an example, if the result of Test #1 is positive, the subject material is directed to Test Area #2 by a first gas generator, where a subsequent analysis is performed based on the result of Test #1. Conversely, if the result of Test #1 is negative, then the subject material is directed to Test Area #3 by a different gas generator.
As previously noted, one advantage of the invention is that only a minuscule amount of gas-generating compound is needed to provide sufficient decomposition to generate enough gas to drive the subject material to the next test area of the device. Moreover, since the amount of gas-generating compound is small, the energy required to decompose the compound can be easily localized to the point of decomposition. The heat generated by the resistors is therefore unlikely to cause damage to the subject material.
Optionally, each gas-forming chamber may include an array of discrete volumes of the gas-generating compound. A corresponding number of microheaters may be provided, so that the microheaters of a particular chamber can be activated individually or in unison to generate a precise quantity of gas. As an example, eight binary-weighted quantities of gas-generating material can be contained within a single gas generator. By appropriate activation of the eight associated microheaters, one of 256 different quantities of gas can be generated. This embodiment may be used in situations in which the required pumping forces must be carefully controlled. Alternatively, the precise quantity of gas can be generated by analog control. That is, rather than controlling the quantity of gas-generating compounds that is selectively heated to generate the precise quantity of gas, the specific amount of heat that is applied to the gas-generating compounds is controlled.
As such, a precise quantity of gas is generated in relative proportion to the amount of heat that is applied.